Re: [PATCH v3 0/5] enhance DMA CMA on x86

[Akinobu Mita]

2014-10-04 1:39 GMT+09:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
> On 10/03/2014 12:06 PM, Akinobu Mita wrote:
>> 2014-10-03 23:27 GMT+09:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
>>> On 10/02/2014 07:08 PM, Akinobu Mita wrote:
>>>> 2014-10-03 7:03 GMT+09:00 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>:
>>>>> On 10/02/2014 12:41 PM, Konrad Rzeszutek Wilk wrote:
>>>>>> On Tue, Sep 30, 2014 at 09:49:54PM -0400, Peter Hurley wrote:
>>>>>>> On 09/30/2014 07:45 PM, Thomas Gleixner wrote:
>>>>
>>>>>>> Which is different than if the plan is to ship production units for x86;
>>>>>>> then a general purpose solution will be required.
>>>>>>>
>>>>>>> As to the good design of a general purpose solution for allocating and
>>>>>>> mapping huge order pages, you are certainly more qualified to help Akinobu
>>>>>>> than I am.
>>>>>
>>>>> What Akinobu's patches intend to support is:
>>>>>
>>>>>         phys_addr = dma_alloc_coherent(dev, 64 * 1024 * 1024, &bus_addr, GFP_KERNEL);
>>>>>
>>>>> which raises three issues:
>>>>>
>>>>> 1. Where do coherent blocks of this size come from?
>>>>> 2. How to prevent fragmentation of these reserved blocks over time by
>>>>>    existing DMA users?
>>>>> 3. Is this support generically required across all iommu implementations on x86?
>>>>>
>>>>> Questions 1 and 2 are non-trivial, in the general case, otherwise the page
>>>>> allocator would already do this. Simply dropping in the contiguous memory
>>>>> allocator doesn't work because CMA does not have the same policy and performance
>>>>> as the page allocator, and is already causing performance regressions even
>>>>> in the absence of huge page allocations.
>>>>
>>>> Could you take a look at the patches I sent?  Can they fix these issues?
>>>> https://lkml.org/lkml/2014/9/28/110
>>>>
>>>> With these patches, normal alloc_pages() is used for allocation first
>>>> and dma_alloc_from_contiguous() is used as a fallback.
>>>
>>> Sure, I can test these patches this weekend.
>>> Where are the unit tests?
>>
>> Thanks a lot.  I would like to know whether the performance regression
>> you see will disappear or not with these patches as if CONFIG_DMA_CMA is
>> disabled.
>
> I think something may have gotten lost in translation.
>
> My "test" consists of doing my daily work (email, emacs, kernel builds,
> web breaks, etc).
>
> I don't have a testsuite that validates a page allocator or records any
> performance metrics (for TTM allocations under load, as an example).
>
> Without a unit test and performance metrics, my "test" is not really
> positive affirmation of a correct implementation.
>
>
>>>>> So that's why I raised question 3; is making the necessary compromises to support
>>>>> 64MB coherent DMA allocations across all x86 iommu implementations actually
>>>>> required?
>>>>>
>>>>> Prior to Akinobu's patches, the use of CMA by x86 iommu configurations was
>>>>> designed to be limited to testing configurations, as the introductory
>>>>> commit states:
>>>>>
>>>>> commit 0a2b9a6ea93650b8a00f9fd5ee8fdd25671e2df6
>>>>> Author: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
>>>>> Date:   Thu Dec 29 13:09:51 2011 +0100
>>>>>
>>>>>     X86: integrate CMA with DMA-mapping subsystem
>>>>>
>>>>>     This patch adds support for CMA to dma-mapping subsystem for x86
>>>>>     architecture that uses common pci-dma/pci-nommu implementation. This
>>>>>     allows to test CMA on KVM/QEMU and a lot of common x86 boxes.
>>>>>
>>>>>     Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
>>>>>     Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
>>>>>     CC: Michal Nazarewicz <mina86@xxxxxxxxxx>
>>>>>     Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
>>>>>
>>>>>
>>>>> Which brings me to my suggestion: if support for huge coherent DMA is
>>>>> required only for a special test platform, then could not this support
>>>>> be specific to a new iommu configuration, namely iommu=cma, which would
>>>>> get initialized much the same way that iommu=calgary is now.
>>>>>
>>>>> The code for such a iommu configuration would mostly duplicate
>>>>> arch/x86/kernel/pci-swiotlb.c and the CMA support would get removed from
>>>>> the other x86 iommu implementations.
>>>>
>>>> I'm not sure I read correctly, though.  Can boot option 'cma=0' also
>>>> help avoiding CMA from IOMMU implementation?
>>>
>>> Maybe, but that's not an appropriate solution for distro kernels.
>>>
>>> Nor does this address configurations that want a really large CMA so
>>> 1GB huge pages can be allocated (not for DMA though).

kernel parameter 'cma=' is only available when CONFIG_DMA_CMA is enabled.
cma=0 doesn't disable 1GB huge pages as far as I can see.
So I prepare a patch which make default cma size zero on x86.

>> Now I see the point of iommu=cma you suggested.  But what should we do
>> when CONFIG_SWIOTLB is disabled, especially for x86_32?
>> Should we just introduce yet another flag to tell not using DMA_CMA
>> instead of adding new swiotlb-like iommu implementation?
>
> Again, since I don't know what you're using this for and
> there are no existing mainline users, I can't really design this for
> you.
>
> I'm just trying to do my best to come up with alternative solutions
> that limit the impact to existing x86 configurations, while still
> achieving your goals (without really knowing what those design
> constraints are).

Thanks a lot for your advise.
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